Split liquid separations

Assume initially that a phase split can separate the reactor effluent into a vapor stream containing only hydrogen and methane and a liquid stream containing only benzene, toluene, and diphenyl and that the liquid separation system can produce essentially pure products. 

The heated oil is flashed into the VPS flash zone where vapor and liquid separate. Split between distillate and bottoms depends on flash zone temperature and pressure. Separation of vapor and liquid in the flash zone is incomplete, since some lower boiliug sidestream components are present in the liquid while bottoms components are entrained in the vapor. The liquid from the flash zone is steam stripped in the bottom section of the VPS to remove the lower boiling components. 

If the liquid from the phase split requires separation, then this can normally be accomplished by distillation, except under special circumstances. A distillation sequence is most often required with products and byproducts removed from the process and unreacted feed material recycled. In some situations, byproducts might be recycled for reasons discussed in the previous section. 

The liquid stream can readily be separated into relatively pure components by distillation, the benzene taken off as product, diphenyl as an unwanted byproduct and the toluene recycled. It is possible to recycle the diphenyl to improve selectivity, but it will be assumed that is not done here. The hydrogen feed contains methane as an impurity at a mole fraction of 0.05. The production rate of benzene required is 265 kmol-lr1. Assume initially that a phase split can separate the reactor effluent into a vapor stream containing only hydrogen and methane, and a liquid containing only benzene, toluene and diphenyl, and that it can be separated to produce essentially pure products. For a conversion in the reactor of 0.75,... 

When a mixture in a reactor effluent contains components with a wide range of volatilities, then a partial condensation from the vapor phase followed by a simple phase split can often produce a good separation. If the vapor from such a phase split is difficult to condense, then further separation needs to be carried out in a vapor separation process such as a membrane. The liquid from the phase split can be sent to a liquid separation unit such as distillation. 

The reactor/separator/recycle structure is decided by considering the physical properties of the species found in the reactor effluent (Table 9.1). The catalyst and the organic phase are immiscible. Therefore, they can be separated by liquid-liquid splitting. The separation of the organic components by distillation seems easy. In a direct sequence, the inert and any light byproduct will be removed in the first column. The second column will separate the reactants, which have adjacent volatilities. Therefore, there will be only one recycle for both reactants. The third column will separate the product from the heavies. The reactor/separation/ recycle structure of the flowsheet is presented in Figure 9.2. 

With increase in temperature there occurs increase in violence of molecular motion of all kinds. Gas molecules rotate more rapidly, and the atoms within a molecule oscillate more rapidly relative to one another. The atoms and molecules in liquids and solids carry out more vigorous vibrational motions. This vigorous motion at high. temperatures may result in chemical reaction, especially decomposition of substances. Thus when iodine gas is heated to about 1200° C at 1 atm pressure about one-half of the molecules dissociate (split) into separate iodine atoms (Fig. 3-17). 

Wahnschafft, O. M., LeRudulier, J. P., Blania, P., and Westerberg, A. W. SPLIT II. Automated Synthesis of Hybrid Liquid Separation Systems, Comput. Chem. Eng. 16, S305-S312 (1992). 

Flash2 - rigorous vapor-liquid split or vapor liquid liquid split FlashS - rigorous vapor-liquid-liquid split Decanter - separate two liquid phases Sep - use split fractions... 

Dynamic flash is a simple but very useful unit in dynamic simulation. Fig. 4.2 depicts the layout of a vapour-liquid separation. A multi-component feed of molar flow rate F with the composition z, is split in vapour V and liquid L, with the composition y-, and Xj, respectively. Optionally heat may be added or removed. Initially the flash operates at steady state. The problem is to study the dynamic response at various disturbances, as changes in throughput or composition. Modelling equations are presented below. 

A heterogeneous gas/liquid reactor illustrates a two-phase effluent (Fig. 7.13). The two phases are already present inside the reaction space. Vapour phase may undergo a new phase-split after condensation. The secondary vapour enters the vapour separation system. Gaseous reactants are recycled to the reactor, but purge may be necessary to eliminate gaseous products or avoid the accumulation of inert. The liquid streams from phase split and vapour recovery are sent to the liquid separation system, from which the liquid reactant is recycled. 

Firstly, the mixture must be condensed and split in gas and liquid phases in a flash vessel (Fig. 7.14). The condensable components are sent to the liquid separation system, while the non-condensable components are treated in the gas separation system. Another solution is applying a quench to the reactor outlet with recycled solvent. 

The phase-split block can be a single flash, a series of flashes, or a combination of flash and absorption/stripping columns. Flash temperature and pressure are design variable that may be optimised to fulfil a separation objective, as sharp gas/liquid split or recovery of some components. For water-driven condensers the recommended condensation temperature is of about 35 °C. Vapour components can be condensed and sent to the liquid separation system. The supercritical components carried in the liquid phase can be recovered in a stabiliser column (see later in this section). Further, these can be sent to the gas separation system, used as fuel, or purged. 

At higher IL concentrations it is not possible to induce a liquid-liquid separation by the addition of CO2, i.e., no LCEP is observed [17]. This complex phase behavior has certain implications for the use of these biphasic systems as reaction/separation media. If large concentrations of reactants and products are present, relative to the IL itself, an additional liquid phase might occur. A liquid-liquid phase split may then be detrimental in homogeneously catalyzed reactions, introducing additional mass-transport limitations. 

In Figure 1 a simplified process scheme of the antisolvent crystallization of sodium chloride is displayed. The process is divided into three steps the crystallization, the solid-liquid separation and the antisolvent recovery or liquid-liquid separation. In the first step sodium chloride is crystallized by mixing the feed brine with an antisolvent. The crystallization is carried out at temperatures below the liquid-liquid equilibrium line in the single liquid phase area (see Figure 2). In the second step the crystals are separated from their mother liquor, e.g. by filtration or in a centrifuge. In the third and final step the antisolvent is separated from the water phase at a temperature above the liquid-liquid equilibrium line in the two liquid phase area, in which the ternary amine-water-salt system splits up into an amine and an aqueous phase. The recovered antisolvent is recycled within the process and most ideally the water phase is reused for the dissolution of crude sodium chloride. In this paper the crystallization and the liquid-liquid separation steps will be treated. 

In Fig. 10.15, concentration and temperature profiles along the colurim are presented for a selected experiment. The average deviation for conversion and product purity is about 2%. For the experiments with KATAPAK -S, no liquid-liquid phase separation was observed at the laboratory scale, mainly because there was still too much ethanol in the distillate fraction preventing the phase splitting. The reason is that the catalytic section is only 1 m high and the catalyst fraction of KATAPAK -S Lab is lower as compared to MULTIPAK -2 or the industrial KATAPAK -S 250.Y (see Table 10.3). It was possible to achieve the phase separation in the liquid-liquid separator with the laboratory-scale setup. On the other hand, the size of the catalytic section at the pilot scale enabled even higher conversions and thus liquid-liquid separation, due to the small ethanol fraction in the distillate. 

If for some reason the cyclone discharge must be against back pressure, it may still be worthwhile to maintain the balance between the underflow and overflow. In some applications such as when two or more cyclones are used in series, the back pressure may not be the same on both outlets (in the case of the liquid-liquid separation cyclones for example), and this is quite acceptable provided that the pressures are maintained in the same ratio because this has a profound effect on the flow split in the cyclone and the operating cut size. 

In an equilibrium separation, a feed stream containing m components at given composition, pressure, and enthalpy (or temperature if in a single phase) is split into two streams in equilibrium, here taken to be a vapor and a liquid. The flow rates of the feed, vapor, and liquid streams are, respectively,... 

The same fundamental development as presented here for vapor-liquid flash calculations can be applied to liquid-liquid equilibrium separations. In this case, the feed splits into an extract at rate E and a raffinate at rate R, which are in equilibrium with each other. The compositions of these phases are... 

When a mixture contains components with a broad range of volatilities, either a partial condensation from the vapor phase or a partial vaporization from the liquid phase followed by a simple phase split often can produce an effective separation. This is in essence a single-stage distillation process. However, by its very nature, a single-stage separation does not produce pure products hence further separation of both liquid and vapor streams is often required. 

For a binary, let s denote as V the fractional molar split of the feed into overhead product and as L the fractional split into bottom product. Calculate compositions of the flash separation of feed into vapor v and liquid 1 to give v/1 = V/L. The resulting vapor can be regarded as being composed of a portion d of the overhead product composition and a portion r of the flash liquid composition. 

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